Boost converter 12/48V, UC3843, hissing problem / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Boost converter 12/48V, UC3843, hissing problem

1 2 next

Move Thread

LAN_403

Dr. Dark Current

Thu Jul 21 2011, 10:40PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Hello,
I designed and built this converter, it's a dc motor driver with current limiting. So far I only tried loading it with resistors up to 18W. The problem is that the converter emits squealing and hissing noises, by closer inspection of the gate drive waveform it appears to be "skipping" pulses - one pulse is high for the whole cycle duration, next pulse gets cut right in the beginning (if you get what i mean)...

Any ideas how to fix this?

Patrick

Thu Jul 21 2011, 11:39PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

your measuring the gate wave form at pin 6 right?

Lets check the obvious first...

"Differences between members of this family are the
Â· Low R under-voltage lockout thresholds and maximum duty O Error Amp
cycle ranges. The UC1842 and UC1844 have UVLO
thresholds of 16 VON and 10 VOFF, ideally suited to
off-line applications. The corresponding thresholds
for the UC1843 and UC1845 are 8.4 V and 7.6 V.
The UC1842 and UC1843 can operate to duty cycles
approaching 100%. A range of zero to 50% is
obtained by the UC1844 and UC1845...."

for your version are you above the UVLO?

EDIT: i think your above the UVLO, so what else could it be....

(1)Ok, have you scoped your R and C timing components? Maybe youll see something abnormal?
(2)Have you used your CT and its support circuit before, is it known to be reporting current accurately?

Dr. Dark Current

Fri Jul 22 2011, 08:56AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Patrick,
1. I just checked this, the timing waveform is OK, a nice steady triangle/saw wave.
2. Similar circuit is used in some types of welding inverters, some friends succesfully implemented it in high power variable power supplies.

It appears this problem is pretty common in hobbyist constructions.

Just to say, the regulation works well and nothing seems to heat up so far, I would just like to fix this before I move on higher power testing...

Patrick

Fri Jul 22 2011, 09:04AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

The first pulse goes through, but the next one gets killed... that seems to indicate a problem that is convincing the IC to drag the gate drive pin low.

Dr. Dark Current

Fri Jul 22 2011, 09:32AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Sometimes this problem can happen when a standard rectifier diode is used and when the MOSFET turns on, a reverse recovery current spike of the diode kills the next pulse instantly (as the IC is current controlled and thinks its time to switch the pulse off).
But as I use a schottky rectifier with virtually non-existent reverse recovery time, I can't see how this could be the problem.

Jrz126

Fri Jul 22 2011, 11:46AM

Registered Member #242 Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210

Dr. Kilovolt wrote ...

Hello,
I designed and built this converter, it's a dc motor driver with current limiting. So far I only tried loading it with resistors up to 18W. The problem is that the converter emits squealing and hissing noises, by closer inspection of the gate drive waveform it appears to be "skipping" pulses - one pulse is high for the whole cycle duration, next pulse gets cut right in the beginning (if you get what i mean)...

Any idea where the noise is coming from? I had a similar problem with a SMPS design for work. At low load, it would start hissing, I found it was coming from a SMT tantalum capacitor, (would change the pitch when I pushed on it).

How's the output look? regulating ok? Current waveform look good? Check the CT feedback, maybe there's a spike on the feedback causing it to go into current limit, giving you the shorter second pulse.

Dr. Dark Current

Fri Jul 22 2011, 02:37PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Jeff,
The noise is most likely coming from the inductor and/or capacitors. But that's just a symptom, the cause bad output of the IC.

The voltage is regulating well, so far I tried loading it from a few Watts to 18W and the output voltage changed 1% max.

There are some smaller spikes on the current feedback input, but I don't know if they are real or just the scope picking noise. If they are real, how do I get rid of them? Does a Schottky rectifier also have a reverse recovery current spike?

EDIT: Is continuous or discontinuous current mode better for this kind of converter?

Jrz126

Sat Jul 23 2011, 12:06PM

Registered Member #242 Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210

Can you try running with the current limit disabled? that'll indicate if the current limit is causing the problem.

Dr. Dark Current

Sat Jul 23 2011, 12:28PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

You mean the current sense with the PNP transistor? I already tried disconnecting that without any effect, the current limit is set for ~1.7A and I was drawing 0.37 amps.

Electra

Sat Jul 23 2011, 10:40PM

Registered Member #816 Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156

Is slope compensation required in a boost convertor? Sorry to be a bit vague but only used this sort of ic a little bit, and that was in a flyback mode convertor.
Think this is relevant when the duty cycle is above 50%.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob